This invention relates to the art of metal cutting or shearing and has particular relationship to knives for cutting such metal while it is at high temperatures. In the typical shearing of metal, the part to be cut is interposed between the pair of knives, and the knives are brought together so that the part is sheared between their cutting edges. The knives may be referred to as upper and lower knives. In one practice which is followed, the part to be cut is placed on the surface of the cutting edge of one knife, the lower knife, generally at right angles to the knife, i.e., generally parallel to the surface. The part extends over this surface with the line along which the cut is to be made along the cutting corner of the knife. A second knife, the upper knife, reacts with the lower knife to shear the part. One knife is moved relative to the other or both knives are moved. For example, the upper knife is moved downwardly relative to the first knife with the cutting corner of the second knife parallel to the cutting corner of the first knife. The cutting corner of the upper knife moves through the part to be cut near the cutting corner of the first knife. The part to be cut is sheared between the cutting corners of the two knives. The lower knife may move relative to the upper knife or both knives may move together.
Typically, this invention concerns itself with the cutting of hot metal such as blooms, billets, slabs or bars with knives which operate as described above. A bloom is a semi-finished product which is rolled from an ingot. While being cut the bloom is at temperatures between 1800.degree. F. and 2200.degree. F. The temperature may, however, be at times as low as 1400.degree. F. and as high as 2400.degree. F. Such knives are also used in cutting billets from strands produced by continuous casting. In this application, the expression "high temperature" means generally a temperature between about 1400.degree. F. and 2400.degree. F., but predominately between 1800.degree. F. and 2200.degree. F.
In accordance with the teaching of earlier prior art, knives of alloy tool steel were used in cutting metal at high temperatures. Knives of alloy tool steel in this service had a relatively short life in continuous use, typically about two weeks notwithstanding that they were water cooled during the shearing. The users of these knives were plagued by the high cost of the numerous knives which were required in their operations, by the down time and by the cost of continually replacing the knives.
More recently the practice has been adopted of using knives made of so-called super alloys or high-temperature alloys such as INCONEL Alloy 718. This alloy is high-temperature resistant and knives of this alloy have a service life in continuous use before requiring regrinding of many weeks, typically of three months or more. The expression "high-temperature resistant" as used in this application means resistant to temperatures substantially exceeding 2000.degree. F. As applied to an alloy, this expression means that a knife composed of this alloy when used continuously for cutting metal at high temperatures has a life, before requiring regrinding, of several months or longer. In this kind of service this alloy must be subjected to heat treatment (age hardening) before being used. INCONEL Alloy 718 has substantially the following nominal composition in weight percent:
______________________________________ Nickel 52.5 Chromium 19.0 Molybdenum 3.0 Columbium 5.1 Aluminum 0.5 Titanium 0.9 Iron 18.5 Manganese 0.2 Silicon 0.2 Carbon 0.04 ______________________________________
INCONEL Alloy 718 is costly. Typically it costs in excess of $9.00 per pound. A typical blank for a knife weighs about 1300 pounds having a cost in excess of $11,7000.00. The fabrication costs to make this knife are substantial. A knife of INCONEL alloy ready for use may cost about $17,000.00 or more.
It is an object of this invention to overcome the drawbacks of the prior art and to provide a high-temperature resistant knife for cutting metal at high temperatures which, while not excessively costly, will have a long life before requiring regrinding. There is an advantage in addition to the cost advantage in achieving this object. The achievement will substantially conserve important alloying elements such as molybdenum, columbium, nickel, chromium and others which are urgently needed in the aircraft industry particularly for defense.